Fuel tank inerting systems (also called flammability reduction systems) are used on most commercial aircraft. They are used to provide an inert atmosphere to the ullage or header of a fuel tank in order to reduce the risk of fuel combustion and explosion.
Typically they employ air separation membrane technology. However, such known systems degrade over time and require significant levels of costly maintenance. An alternative system is a catalyst based fuel tank inerting system, where bleed-air-fed air separation modules and filter elements are replaced by a catalyst. Such a system requires fuel vapour to be mixed with oxygen and passed through a catalyst device to be converted into carbon dioxide (CO2) and water (H2O). Due to significant temperatures that are generated during the catalyst's exothermic reaction the catalyst needs to be cooled. Hence the catalyst may be provided as a coating inside a heat exchanger. The heat exchanger may be cooled by e.g. ram air.
However, in use such heat exchangers face issues because different amounts of cooling are available during different stages of flight. For example, operating conditions on the ground are different to those at cruise. Ram air at cruise can have a temperature of −40° F. (−40° C.), whereas on the ground in hot climates it can have a temperature as much as +130° F. (50° C.). The catalysed reaction of fuel and oxygen in the heat exchanger must be kept at a minimum of 350° F. (180° C.) in order to keep the catalyst ‘lit’ (i.e. reacting) and the catalysed reaction active. A heat exchanger sized so that it achieves a maximum temperature of 425° F. (220° C.) on the ground will produce warmed ram air of 1600° F. (870° C.) during cruise, far in excess of the necessary temperatures. Making a heat exchanger small in order to avoid such high cruise temperatures results in too high ground temperatures. Thus, it is necessary to compromise and use a heat exchanger that is non-optimal for all conditions.